Poison dart frogs are part of the family
Dendrobatidae (8). These colorful dwellers of rainforests and other warm
moist climates are called poison dart frogs or poison arrow frogs because
South American Indians use the potent toxin secreted from granular skin
glands on the back of some frogs to poison darts used for hunting (3, 8,
10). In the mid 1960s, this toxin known as batrachotoxin, was discovered in
skin extracts of Columbian frogs in the family Dendrobatidae (5). Of the
many poisonous frogs, only those from the genus Phyllobates secrete
batrachotoxin. High levels of this toxin are found in the western Columbian
Phyllobates terribilis, Phyllobates bicolor, and Phyllobates aurotaenia
while trace amounts are found in the Central American Phyllobates vittatus
and Phyllobates lugubris (4). The Noanama, Choco and Cuna Indians of
Columbia use batrachotoxins from the three frogs with the most toxins to
poison the tips of darts used for hunting (7).

Picture of Phyllobates terribilis (6)

Batrachotoxin is a steroidal alkaloid found in
the granular skin glands of frogs in the genus Phyllobates (1). It protects
the frogs by producing noxious effects in the mouths of predators (3).
Although the toxin helps protect them, some snakes and large spiders are
predators of adult frogs. Most other animals cannot tolerate the poison
secreted from the skin glands. Predators of tadpoles, which are unprotected
by toxins, include snakes and dragonfly larvae (8). Batrachotoxin is very
potent and selective (1). The skin of Phyllobates terribilis contains about
1mg of batrachotoxin: more than any other species in that genus (5). This
toxin is not produced by the frogs themselves, but is instead taken up
through diet and secreted through the skin of the frog (6).
Structure of batrachotoxin (9).

John W. Daly, a leading expert in this field,
studied captive frogs of the family Dendrobatidae, which did not produce
toxins initially. These frogs were fed a diet of arthropods containing
alkaloids. The alkaloids were accumulated unchanged in the skin of the
experimental frogs (6). Usually captive frogs are not toxic because the
alkaloid rich diet those in the rainforest normally eat is not available to
the average pet owner (4).

Because of their diets, Phyllobates terribilis
and the other species of poisonous frogs are toxic in the wild (5). The
toxin found in these frogs, batrachotoxin, is very potent and specific.
According to Jiri Patockaa and Ladislav Stredab, at doses less that 0.1 µg,
symptoms observed in laboratory animals include convulsions, salivation,
muscle contractions, and death. In mice, the subcutaneous LD50 is 0.2 µg/kg;
though doses as low as 0.01 and 0.02 µg/kg have proven lethal (9). Myers et
al. estimated a lethal injection dose of 2.0 to 7.5 µg for humans. When
administered orally, the toxicity is much lower, making safe for Indians to
consume animals hunted with the poison darts. The toxins may also be
destroyed by cooking (9). Other symptoms of intoxication include muscle and
respiratory paralysis, strong muscle contractions, convulsions, salivation,
and death (7, 9).

In their book Principles of Toxicology, Thomas
Brown and Karen Stine state that batrachotoxin acts by preventing
voltage-gated sodium channels from closing in nerves. An action potential is
not allowed to developed, and the nerve impulse is not sent on. Sodium
channels are also made less selective; therefore, ions other than sodium can
enter the cell through the channels (2). This toxin has been used to study
the function of sodium channels because it is both potent and specific (5).
Interestingly, tetrodotoxin is a noncompetitive inhibitor of this sodium
channel activation. It binds to a separate site on the channel and acts as
an antagonist (1). John W. Daly states that Phyllobates are not susceptible
to the alkaloids because the sodium channels in their nerves and muscles are
modified (3).

Poison dart frogs are not the only species that
contain batrachotoxins. Some birds found in New Guinea as well as insects
there and in South America also have been shown to contain this toxin. Like
Phyllobates, the birds are thought to consume the toxin and secrete it
through their skin. New research suggests that Choresine beetles are the
source of batrachotoxin for the birds while relatives of this arthropod
might be the source for frogs (5).

Nature has given these frogs a way to protect
themselves. While beautifully colored and seemingly harmless, these toxic
creatures secrete a very toxic substance from their skin. When traveling to
South and Central America stay clear of these dangerous frogs.

References:
(1) Bartels-Bernal, Eva, Terrone L. Rosenberry, and John W. Daly. “Effect of
batrachotoxin on the electroplax of electric eel: Evidence for
voltage-dependent interaction with sodium channels.” Proceedings of the
National Academy of Science of the United States of America. Vol. 74, No. 3,
pp. 951-955, March 1977 <http://www.pnas.org/cgi/reprint/74/3/951>

(4) Dumbacher John P., Thomas F. Spander, and John W. Daly “From the Cover:
Batrachotoxin alkaloids from passerine birds: A second toxic bird genus (Ifrita
kowaldi) from New Guinea” Proceedings of the National Academy of Science of
the United States of America. Vol. 97: 12970-12975. <http://www.pnas.org/cgi/content/full/97/24/12970>

(5) Dumbacher, John P., Avit Wako, Scott R. Derrickson, Allan Samuelson,
Thomas F. Spander, and John W. Daly. “Melyrid beetles (Choresine): A
putative source for the batrachotoxin alkaloids found in poison-dart frogs
and toxic passerine birds.” Proceeding of the National Academy of Sciences
of the United States of America 2004 November 9; 101(45): 15857–15860.
(2004). 5 December 2005 <http://www.pubmedcentral.gov/articlerender.fcgi?tool=pubmed&pubmedid=15520388#top>